riscv64 is a LP64_ARCH, list it as such.

Author: drahn

Makefile.cross

@@ -1,4 +1,4 @@
-# $OpenBSD: Makefile.cross,v 1.110 2021/05/01 16:10:29 visa Exp $
+# $OpenBSD: Makefile.cross,v 1.111 2021/05/03 00:04:31 drahn Exp $
 
 cross-tools:	cross-includes cross-binutils cross-gcc cross-lib
 cross-distrib:	cross-tools cross-bin cross-share cross-sys cross-etc-root-var
@@ -28,7 +28,7 @@ TARGET_CPU?=	${TARGET_ARCH}
 
 # List of LP64 archs
 # XXX move to bsd.*.mk?
-LP64_ARCH = aarch64 amd64 alpha mips64 mips64el sparc64 powerpc64
+LP64_ARCH = aarch64 amd64 alpha mips64 mips64el riscv64 sparc64 powerpc64
 
 TARGET_IS_LP64 =
 MACHINE_IS_LP64 =